SR250 - Rebuild engine questions

Hi, I'm new to the forum. I just bought a year a go a Yamaha SR 250, it's a pretty common bike here in Spain (It has been fabricated in Barcelona in a old Sangas factory)

I've been cleaning it and repairing what I'm able to but just recently I did a compression test and it gives me around 120 psi (Jadus said that is a pretty bad result knowing the fact that it should make like 180 psi)

I did some test to check whats wrong with it and I know for sure that the exhaust valve it's not in good condition (I hold a paper on the exhaust and its blown and sucked)

I have enought time to rectificate the engine but I have a bunch of questions:

[list type=decimal]
[*]Should I change the piston or just the rings?
[*]Should I rectifie the cylinder?
[*]How can I obtain moar power? i hear that using a yamahas xt250's piston can help a little bit
[*]Do you have any recomendation on the matter?
[/list]

Thanks

There are some SR250 experts on here that can help you with more specific questions but:

1. This depends. Your piston OD (outer diameter) must be measured as well as the ID (inside diameter) of the cylinder, and the cylinder should also be checked to make sure it is not out of round. The service manual will have a range for piston to cylinder clearance. If it is out of spec, you will need to replace the piston and bore the cylinder. If it is within spec, just replace the piston rings.

2. Again this depends on the measurements you get for the piston & cylinder. If out of spec it will need to be bored to the next size up and matched an oversize piston. If in spec it can be honed with a ball hone to put a cross hatch on the cylinder and help the rings seat.

If you do not have the proper measuring tools to check piston & cylinder measurements, bring it to an automotive machinist and they can measure for you.

Yeah I'm gonna check the cylinder and the piston an probably rectifie the cylinder, It problably needs it before 30 years of use.

I heard on one thread that you can use Yamaha Raptor's piston but I'm not sure if that would be healthy for the engine

Thanks